It is well founded that exfoliation of epidermal layers of human skin induces an increased rate of epidermal cell renewal (E. Phillips, 1995, U.S. Pat. No. 5,431,913; W. P. Smith, 1994, Cosmetics and Toiletries 109:41-8). The human epidermis consists of multiple layers of stratified squamous epithelial cells in a constant state of renewal. New cells are formed first in the basal layer, which is the most internal membrane of the epidermis. These cells are displaced by the production of yet newer cells and subsequently are transported to the external layer of the epidermis, the stratum corneum, where they usually are shed (exfoliated) every two to three weeks. The general health and appearance of human skin depends greatly upon the rate of this process.
Certain situations or conditions, such as aging or exposure to the environment, can disturb this normal process and can lead to a generally reduced rate of cell renewal as well as an increased degree of intercorneocyte cohesion (VanScott and Yu, 1984, J. Am. Acad. Dermatol. 11:867-79). Because the time required for a cellular layer to migrate from the basal layer to the stratum corneum increases with the subject's age, the rate of epidermal cell renewal decreases. It has been reported that in a typical twenty year old person, the cells in the outer layers of the epidermis turn over on the average, every two weeks, while cell turnover intervals of more mature skin can be as much as twice as long (E. Phillips, 1995, U.S. Pat. No. 5,431,913). The decrease in cell renewal rate due to aging can be exacerbated by environmental conditions such as exposure to solar radiation and other climatic conditions (K. E. Burke, 1990, Postgraduate Medicine 88(1):207-27). A decrease in the cell renewal rate increases the time cells in the outer layers of skin are exposed to environmental conditions and may lead to further and/or cumulative damage. Certain studies suggest that a decrease in epidermal cell turnover rates is associated with an increase in intercorneocyte cohesion (VanScott and Yu, 1989, Cutis 43:222-28).
The cells of the epidermal layer are held together by proteinaceous components such as hemidesmosomes, desmosomes, gap junctions, glycosaminoglycans, proteoglycans, and other components present in the skin all of which have the ability to bind to each other and to cellular components. This binding together of the epidermal cells is described as "intercorneocyte cohesion", and the degree of the cohesive strength involved is affected by such factors as hydration and pH. Increased intercorneocyte cohesion results in hyperkeratinization and is characterized by thick and often dry or scaly skin caused by the retention of epidermal cells. The balance that exists between cohesion and cell turnover rates is responsible for normal rejuvenation of young skin and imbalances of these factors can result in the aged, rough, and unattractive look of mature epidermal surfaces. The search for topically active components that balance cell renewal rates and intercorneocyte cohesion is prominent in dermatological research efforts today (VanScott and Yu, 1984, J. Am. Acad. Dermatol. 11:867-79).
Recent advances in this research area have provided a number of topically active acidic compounds which show promise in this regard (Yu et al., 1978, U.S. Pat. No. 4,105,783; Yu et al., 1982, U.S. Pat. No. 4,363,815). Long term treatments with these acidic compounds result in an increased rate of epidermal cell renewal. Those acidic components known to be epidermally active, such as low molecular weight hydroxy or keto acids and esters thereof, are well documented with respect to their effectiveness as keratolytic/desquamation agents. High concentrations of these acids (e.g., salicylic and glycolic acids), as well as other acids such as trichloroacetic acid, are known for their ability to cause destruction of tissue at the site of application (W. L. Epstein, 1990, in Irritant Contact Dermatitis, Jackson and Glodner, eds., Marcel Decker, Inc., New York and Basel, pp. 127-165; Remington's Pharmaceutical Sciences, A. Osol, ed., Sixteenth Ed., Mack Publishing, Inc., Easton, Pa., 1980). At lower concentrations, these acids have been shown to have the ability to loosen the dead cells in the keratin-rich stratum corneum and interrupt intercorneocyte cohesion thereby facilitating desquamation; this activity is called "keratolytic activity" (Remington's Pharmaceutical Sciences, A. Osol, ed., Sixteenth Ed., Mack Publishing, Inc., Easton, Pa., 1980; W. P. Smith, 1994, Cosmetics and Toiletries 109:41-8).
Most keratolytics are skin irritants, including those listed above. Although some keratolytics are promoted as being more efficient than others, a comparison of the therapeutic index of each of these acids shows little advantage of one over another. The "therapeutic index" is an accurate method of rating the efficiency of these components which takes into account the component's keratolytic efficiency as well as the levels of irritation at a given concentration. It has been shown that even low levels of these components (greater than 4% acid concentration) cause significant skin irritation. The use of even lower levels of these components reduces irritation but generally reduces keratolytic effectiveness. Furthermore, prolonged use of the acids reduces the efficiency of cell renewal induction (W. P. Smith, 1994, Cosmetics and Toiletries 109:41-8). These shortcomings point out the need for a methodology to enhance the keratolytic effectiveness of these acids at non-irritating concentrations.
The application of proteolytic enzymes in topical therapy has been used for some time for such things as scar removal from burn wounds and as an adjunct to anti-microbial therapy (G. Rodeheaver, 1975, Am. J. Surg. 129(5):537-544). These enzymes include those generally restricted to plant sources, such as papaya (papain), fig (ficin), and pineapple (bromelain). Cosmetic formulations containing extracts from these plants and promoted as enhancing the removal of outer epidermal layers by proteolytic action have been marketed commercially. Although the substrate specificity and spectrum of activity in relation to pH suggest that the proteolytic action of these formulations should result in keratolytic activity at the outer epidermal layers, applications of proteolytic enzymes previously used in topical therapy have not been without drawbacks.
Those enzymes (e.g., papain, bromelain, and ficin) currently used in cosmetic applications generally exhibit proteolytic activity over a broad pH range of pH 3-pH 9 (Glazer and Smith, 1971, in The Enzymes, Vol. 3, P. Boyer, ed., Academic Press, New York, pp. 501-546). It is probably the broad pH range of these plant-derived enzymes that is responsible for at least some of the problems associated with prolonged skin exposure. Human epidermal systems maintain a pH gradient within the stratified layers of the skin; the outer layers have been reported to exhibit an average pH of 5.5 (W. P. Smith, 1994, Cosmetics and Toiletries 109:41-48). The pH of successive layers of the epidermis increases with depth, reaching a final pH closer to the physiological range (about pH 7.4) at the dermal layer. Because the plant-derived enzymes mentioned above remain active across this pH gradient, there is no pH control over these plant enzyme activities when they are applied to the skin.
The degree of therapeutic activity received by topical application of proteolytic enzymes has been governed by the intrinsic catalytic characteristics of those enzymes. The wide range of proteolytic activity, in relation to pH, exhibited by the proteases discussed above (pH 3-9) allows for little or no control over proteolytic activity by product formulation (Glazer and Smith, 1971, in The Enzymes, Vol. 3, P. Boyer, ed., Academic Press, New York, pp. 501-546). The plants from which these enzymes have been obtained are known to cause irritant contact dermatitis with symptoms including itching, edema, and blistering. It has been postulated that these enzymes are the primary cause of those symptoms possibly due to excessive decreases in intercorneocyte cohesion (W. L. Epstein, 1990, in Irritant Contact Dermatitis, Jackson and Glodner, eds., Marcel Decker, Inc., New York and Basel, pp. 127-165). The enzymes currently used in cosmetic applications can exhibit substrate-dependent proteolytic activity throughout all layers of the epidermis. Uncontrolled proteolysis of epidermal proteins that serve to stabilize intercorneocyte cohesion could decrease this intercorneocyte cohesion excessively and cause subsequent irritation. Proteolytic attack at the basal layer of the epidermis, where cell renewal originates, could cause imbalances in the rate of epidermal cell renewal. Blistering and edema could be the result of uncontrolled proteolytic attack by this class of enzymes at the basal membrane. This possibility is supported by the fact that injection of these plant enzymes into the skin of mammals is known to cause edema in the skin (Morimoto et al., 1987, Ensho 7(6):563-567). This finding has been used to produce experimental edema in dermatological research. Furthermore, the symptoms of irritant contact dermatitis listed above are similar to those induced by the most effective concentrations of epidermally active acidic components currently used in topical applications. Therefore, a more controlled approach to proteolytic activity with respect to topical applications should produce a desirable result without these side effects.
Compositions containing a proteolytic enzyme, pepsin, and several acids have been reported for the dissolution of dense necrotic formations seen, for example, with third degree burns (Soviet Union Patent No. 439288, 1974). There is no indication that these compositions are useful in enhancing epidermal exfoliation or that these compositions are active only for limited time periods.
Citation or identification of any reference in the background of this application shall not be construed as an admission that such reference is available as prior art to the present invention.